1. Technical Field
The present invention relates to multiple ratio geared transmissions for use in an automotive vehicle powertrain and to a control strategy for effecting engagement and release of transmission friction torque establishing elements during a ratio change.
2. Background Art
A multiple-ratio (i.e., step-ratio) automatic transmission in an automotive vehicle powertrain adjusts a gear ratio between a torque source and a driveshaft to meet drive-ability requirements under dynamically-changing driving conditions. Ratio changes are achieved by engaging a so-called on-coming clutch (“OCC”) as a so-called off-going clutch is released. The clutches, which may be referred to as transmission friction elements or brakes, establish and disestablish power flow paths from an internal combustion engine to vehicle traction wheels. During acceleration of the vehicle, the overall speed ratio, which is the ratio of transmission input shaft speed to transmission output shaft speed, is reduced as vehicle speed increases for a given engine throttle setting. This is an up-shift.
In the case of a non-synchronous automatic transmission, the up-shift event involves engagement control of only the OCC, while a companion clutch, typically a one-way coupling (“OWC”), automatically disengages to reduce both speed ratio and torque ratio. During the non-synchronous up-shift, the OCC becomes automatically unlocked and overrun to lower both the gear ratio (i.e., the overall speed ratio) and the torque ratio (the ratio of output torque to input torque).
The non-synchronous up-shift can be divided into three phases, which may be referred to as a preparatory phase, a torque phase, and an inertia phase. The torque phase is a time period when the OCC torque is purposely raised for its engagement until the OWC starts slipping or overrunning. The non-synchronous up-shift does not involve active control of the OWC. Torque transmitted through the OWC automatically decreases in response to increasing OCC torque. As a result of this interaction, the transmission output shaft torque drops creating the so-called “torque hole.” A large torque hole can be perceived by a vehicle occupant as an unpleasant shift shock. The preparatory phase is a time period prior to the torque phase. The inertia phase is a time period when the OWC starts to slip, following the torque phase.
A conventional non-synchronous control during the torque phase relies on an open-loop approach for OCC engagement. Such an open-loop approach requires manual adjustment of OCC control parameters under multiple operating conditions, resulting in a large amount of effort to calibrate shift quality. It is also difficult to account for variations in actuator characteristics and dynamically changing operating conditions, resulting in inconsistent shift quality.
Other control techniques employ a coupled engine-transmission control during the torque phase to reduce or eliminate torque holes. However, in practice, it is difficult to coordinate engine torque and clutch engagement due to their finite controllability with the presence of various noise factors. In order to improve the control robustness, some control algorithms aim at reducing errors between target clutch torques as compared with those derived from torque sensor measurements within a transmission system. However, engine and transmission controls still remain tightly coupled through kinematic constraints. Synchronization or coupling between engine torque control and OCC engagement control is still required.
In view of the foregoing, there is a need to reduce the complexity of an up-shift control for improved shift consistency and control robustness.